Hybrid femoral fixation of soft-tissue grafts in anterior cruciate ligament reconstruction using the EndoButton CL and bioabsorbable interference screws: a biomechanical study.

PURPOSE: The purpose of this study was to evaluate the effect of hybrid femoral fixation with bioabsorbable interference screws (BioRCI; Smith & Nephew Endoscopy, Andover, MA) and EndoButton CL (Smith & Nephew Endoscopy) fixation.

METHODS: Biomechanical testing of 3 different fixation techniques was performed by use of porcine hind-limb distal femurs and mature bovine extremity common extensor tendons. Two independent testing sessions were examined. The first testing session (group A) compared femoral fixation via the EndoButton CL device (n = 6) with femoral fixation via the EndoButton CL device with the addition of a BioRCI screw (n = 6). The second testing session (group B) compared femoral fixation via BioRCI screws alone (n = 6) with femoral fixation via the EndoButton CL device with the addition of a BioRCI screw (n = 6). The femur-graft complex was cyclically loaded between 50 and 250 N at 1 Hz for 1,000 cycles. After cycling, the amount of graft slippage was determined by measuring the change in grip-to-grip distance. The complex was then loaded to failure at 1 mm/s, and the ultimate tensile strength, stiffness, and mode of failure were determined.

RESULTS: In group A the addition of an interference screw to the EndoButton CL fixation increased the ultimate tensile strength (1,364.7 +/- 102.4 N for EndoButton CL alone v 1,449.3 +/- 94.4 N for combined technique, P = .035) and stiffness (195.5 +/- 12.1 N/mm for EndoButton CL alone v 307.3 +/- 54.9 N/mm for combined technique, P = .004) and decreased the amount of graft slippage (2.6 +/- 0.5 mm for EndoButton CL alone v 2.0 +/- 0.3 mm for combined technique, P = .017). In group B the addition of the EndoButton CL device to interference screw fixation significantly increased the ultimate tensile strength (643.5 +/- 148.4 N for BioRCI screws alone v 1,290.3 +/- 254.4 N for combined technique, P = .004) but had no effect on stiffness (315.7 +/- 38.9 N/mm for BioRCI screws alone v 341.5 +/- 64.0 N/mm for combined technique, P = .267) or graft slippage (2.7 +/- 1.0 mm for BioRCI screws alone v 2.0 +/- 0.6 mm for combined technique, P = .087).

CONCLUSIONS: Our study shows that hybrid femoral fixation of double-looped gracilis-semitendinosus grafts via the EndoButton CL device and a bioabsorbable interference screw is stronger than interference or EndoButton CL fixation alone with respect to ultimate tensile strength, stiffness, and slippage. The addition of an interference screw to suspensory fixation via the EndoButton CL device increased the ultimate tensile strength from 1,360 N to 1,450 N, improved reconstruction stiffness from 200 N/mm to 300 N/mm, and decreased the amount of graft slippage resulting from cyclic loading from 2.6 mm to 2.0 mm.

CLINICAL RELEVANCE: The hybrid fixation of the EndoButton CL device and an interference screw is a stronger and stiffer construct than either device alone and allows for aperture fixation, which may translate into better clinical results.